Semiconductor laser elements offer very advantageous properties, for example a very reduced light emission surface area, together with a very intense and very collimated laser beam. The optical systems for laser light may therefore be designed with focal distances much shorter than for systems using less collimated light sources, for example incandescent lamps and light-emitting diodes (LEDs). Optical systems for laser light thus have particularly reduced space requirement.
However, the use of a semiconductor laser element as a light source for the lighting modules of automobile vehicles poses certain problems due, notably, to the fact that such a light source emits a substantially monochromatic coherent beam of light. Thus, the type of laser used for lighting or signaling applications onboard an automobile vehicle emits a laser beam which is likely to pose certain safety problems if the laser beam is directly projected onto the road. Such a laser beam could notably be harmful for the eyes of an observer.
Furthermore, the lighting or signaling functions of automobile vehicles require light beams having light with a wider spectrum than that of a laser beam, for example a white light.
In order to solve the safety problems while at the same time transforming the laser beam into light radiation adapted for the lighting or signaling functions, a known solution is to interpose a light-emitting element in the path of the laser beam. Such a light-emitting element comprises, for example, a light-emitting substance which is excited by light whose wavelength corresponds to that of the laser beam, for example blue. As a result, the light-emitting element emits a light whose radiation spectrum extends over a range centered in the yellow and extending from green to red. Thus, at least a part of the incident light of a given wavelength is converted into light which has a wider spectrum of wavelengths and is centered around a wavelength complementary to the initial wavelength and which is emitted in all directions.
In addition, at least another part of the incident light is dispersed by the light-emitting element. In this way, the dispersed light and the converted light are superposed in an additive manner, for example so as to form a white light.
For the aforementioned safety reasons, the light-emitting element is of particular importance. If the light-emitting element were to be damaged or removed from the path of the laser beam, for example following an impact, the concentrated and unconverted laser beam risks being emitted by the lighting module in the direction initially provided for the outgoing light beam. In these cases, safety measures must be envisioned in order to avoid putting the users of the road in danger.
One solution envisioned is to place a device for detection of the wavelength of the laser in the path of the laser beam downstream of the light-emitting element. Thus, when the light-emitting element no longer fulfills its role, the laser beam falls directly onto the detection device. If this is the case, the power supply of the laser element is interrupted by a means for driving the laser element, for example an electronic control unit.
However, such devices require a precise arrangement of the detection device. This notably requires a very small positioning tolerance of the means for guiding the light with respect to the optical means.
Moreover, when it is directly impacted by the laser beam, the detection device is rapidly saturated. It is not therefore very easy to adjust the saturation while at the same time obtaining a detection sensitivity sufficient for the aforementioned safety use.